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98
Chapter 9
Analog to Digital Conversion
AnalogtoDigital and DigitaltoAnalog I/O
Analog refers to physical quantities that vary continuously instead of discretely. Physical phenomena
typically involve analog signals. Examples include temperature, speed, position, pressure, voltage,
altitude, etc. Microprocessors work with digital quantities (values taken from the discrete domain).
For a digital system to interact with analog systems, conversion between analog and digital values is
needed. Building blocks to perform the conversions are: (1) Digital to analog converters (DACs), (2)
Analog to digital converters (ADCs). A digital to analog converter has a digital input that specifies an
output whose value changes in steps. These step changes are in volts or amperes. The analog to digital
converter has an input that can vary from a minimum to a maximum value of volts or amperes. The
output is a digital number that represents the input value.
Transducers
A device that converts a process variable (ex. Car speed) into an electrical signal or vice versa.
Sensors (Input transducers):
Potentiometer (position); strain gauge, piezoelectric device (force);
thermistor,
thermocouple
(temperature);
photoconductive
cell,
phototransistor
(light);
current
transformer, SENSEFET (current);
microphone (sound), etc.
Actuators (Output transducers):
solenoids, relays, speakers; darlington transistors, triacs, etc.
Some common sensors
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Signal Conditioning
Signal conditioning is sometimes necessary because raw sensor outputs are not always suitable for
analogtodigital conversion.
•
Amplification :
Op amps are preferred because of their performance characteristics
Open Loop gain: several hundred thousand, input current ~0, output
impedance ~0.
•
Filtering: LPF
Concepts and terminology
Definitions
•
Offset
: minimum value
•
Span
: maximum value – minimum value
o
Some common spans:
s
range of 0 V to 5 V: span = 5 V
s
range of –12 V to 12 V: span = 24 V
s
range of 4 mA to 20 mA: span = 16 mA
•
Weight:
The analog change corresponding to a change in a bit in the digital number.
•
Step size:
span / 2
n
(Typically, the digital representation is an unsigned
n
 bit integer) (n is
the number of bits in the corresponding digital code. e.g. n=0 for a 1bit number)
•
Resolution
: Same as step size
.
Weight of the LSB.
Example:
Analog signal in range +5 to 5 volts, 8 bit ADC:
Offset = – 5 V
Span = 10 V
Step size = 10 / 256 = 39.1 mV
Notice that the MSB has a weight of 5 V (= span/ 2), and the LSB has a weight of .0391 V
Equations
Analog number = (b
n1
2
–1
+ b
n2
2
–2
+ … +b
0
2
–n
) x stepsize + offset
=
(digital number x step size) + offset
Digital number = (analog number – offset)/ (step size)
These equations hold true as long as the numbers are within the range. If the input of an A/D is below
the minimum or above the maximum of the range, the corresponding digital value will be minimum or
maximum, respectively. In practice, extreme input swing beyond the specified range may damage the
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This note was uploaded on 01/30/2011 for the course EE 447 taught by Professor Skst during the Fall '10 term at Middle East Technical University.
 Fall '10
 skst

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